1. Field of the Invention
The present invention relates to a rotation transmitter that interconnects two rotatable members and transmits mutual rotations of the two members while tolerating an angular misalignment, a parallel misalignment and an axial displacement of rotation axes of the connected members.
2. Description of Related Art
Typically, when two rotatable members are interconnected to transmit mutual rotations by, for instance, extending a rotary shaft or connecting another rotary component to an end of a rotary shaft, various types of rotation transmitters are used. Such a rotation transmitter is called a shaft coupling, joint, or coupling.
The rotation transmitter is required to mutually transmit rotation forces and rotation angle positions between the two connected rotary members as a basic function. Further, when the rotation transmitter is applied to a highly accurate rotation mechanism such as a roundness measuring instrument, the rotation transmitter is required to tolerate an angular misalignment, a parallel misalignment and an axial displacement of the rotation axes of the connected members.
In the roundness measuring instrument, a rotation accuracy of a table on which a workpiece is mounted is enhanced in order to measure roundness of an outer periphery of the workpiece at a high accuracy. In order to rotate the table, a driving shaft for transmitting a rotation force is connected to the table.
Herein, there is inevitably an angular misalignment (an angle of deviation, an inclination of each of central rotation axes), a parallel misalignment (eccentricity, misalignment in an intersecting direction of the central rotation axes) and an axial displacement (deviation of the central rotation axes in an axial direction, axial advance and retraction) between the table and the driving shaft.
When such angular misalignment, parallel misalignment and axial displacement of the rotation axes are directly transmitted from the driving shaft to the table, the rotation accuracy of the table is occasionally affected.
In order to solve the above problem, various rotation transmitters (a universal joint, flexible joint or flexible coupling) capable of reducing or absorbing the angular misalignment, parallel misalignment and axial displacement as described above have been conventionally proposed.
Specifically, examples of the known rotation transmitters include: a so-called disk-type universal joint that uses elastic deformation of a disk in a direction orthogonal to the rotation axis; a cross joint type universal joint that includes two connection pins respectively disposed in a direction orthogonal to the rotation axis and in directions intersecting with each other; and an Oldham universal joint that includes two sets of slide structures, in which each of the slide structures includes a concave component and a convex component that extend in the intersecting directions, and the slide structures are combined in the intersecting direction to transmit rotation.
Among the above rotation transmitters, the Oldham rotation transmitter has a sufficient tolerance to an angular misalignment, a parallel misalignment and axial displacement of the rotation axes.
By using a static-pressure air bearing in the two sets of slide mechanisms in the Oldham rotation transmitter, slide resistance of the slide structures is significantly reduced and a follow-up performance of the slide structures in response to the angular misalignment, parallel misalignment and axial displacement of the rotation axes is enhanced, whereby a more highly accurate rotation transmission is expected to be achievable.
Specifically, there has been proposed a static-pressure rotary joint in which a pair of static-pressure air bearings are provided between inner opposing surfaces of the concave component and outer side surfaces of the convex component, the concave component and the convex component facing each other, in the slide mechanisms of the Oldham rotation transmitter (see Patent Literature 1: JP-UM-3-17058).
In the above static-pressure rotary joint of Patent Literature 1, with the Oldham base arrangement, a sufficient tolerance to the angular misalignment, parallel misalignment and axial displacement is obtained. Moreover, by using the static-pressure air bearing in each of the two sets of slide mechanisms, slide resistance of the slide mechanisms is significantly reduced and follow-up performance in response to the angular misalignment, parallel misalignment and axial displacement is enhanced, whereby a more highly accurate rotation transmission is expected.
However, since the above slide mechanism of the Oldham rotation transmitter is structured such that the convex component is fitted into the concave component so as to be guided in the direction intersecting with the rotation axis, it is difficult to apply the static-pressure air bearing to the rotation transmitter.
Specifically, in the static-pressure air bearing, a slight clearance for forming an air film is necessary between opposing bearing surfaces. This clearance varies depending on operation conditions such as applied loads and supply air pressure.
However, since an inner width of the concave component and a thickness of the convex component are fixed in the above slide mechanism of the Oldham rotation transmitter, the clearance between the concave component and the convex component is also fixed and it is difficult to adjust a dimension of the clearance to an appropriate one as a static-pressure air bearing. Moreover, it is difficult to manufacture the Oldham rotation transmitter having a structure to adjust the dimension of the clearance to an appropriate value.
Because of the adjustment or the difficulty in manufacturing, use of the static-pressure air bearing in the slide mechanism of the Oldham rotation transmitter entails difficulty.
Even when the clearance is adjusted to an appropriate one between the inner bearing surfaces of the concave component and the outer bearing surfaces of the convex component, if the rotation axis is inclined or the like in operation, the convex component is inclined within the concave component, so that a corner or the like of the convex component is possibly brought into contact with the inner surface of the concave component.
In Patent Literature 1 applied to a machine tool, although a smooth operation is disturbed, a processing operation can be maintained. However, when the above slide mechanism is applied to a device of measuring a minute displacement (e.g., a roundness measuring instrument), occurrence of contact or the like of the components severely affects an accuracy of measurement results.
Further, the first member and the second member of which rotation is transmitted via an intermediate member between two slide mechanisms are connected by a static-pressure air bearing without a mechanical connection. Accordingly, in Patent Literature 1, a retainer (fixing members 1 and 15) are provided to support a weight of the intermediate member (distribution members 3 and 17 and joint members 4, 5, 18 and 19).
However, when the intermediate member is supported by such a retainer, a movement (in response to an angular misalignment, a parallel misalignment and an axial displacement of the rotation axis) of the intermediate member is restricted. Accordingly, such an arrangement of using the retainer is not sufficient for the roundness measuring instrument that requires a higher accuracy than the machine tool of Patent Literature 1. Consequently, a high accuracy is possibly not obtained even using the static-pressure air bearing.
Further, in Patent Literature 1, pressurized air is supplied to the static-pressure air bearings of the two slide mechanisms through the retainer. The static-pressure air bearings of the two slide mechanisms rotate together with rotary shafts. In order to externally supply the pressurized air to the rotating static-pressure air bearings, a rotatable connection mechanism is required. The retainer and the intermediate member are used as such a connection mechanism.
Accordingly, the retainer plays a functionally important role in the arrangement of the Patent Literature 1, so that removal of the retainer is difficult.
Thus, the use of the retainer results in failure in obtaining a sufficiently high accuracy of a rotation transmitter for a measuring instrument.